Lng system with optimized heat exchanger configuration

ABSTRACT

The current invention provides a methodology and apparatus for the liquefaction of normally gaseous material, most notably natural gas, which reduces the number of process vessels required and/or reduces space requirements over convention apparatus.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The inventive methodology and associated apparatus disclosed hereinrelates to the liquefaction of a normally gaseous material, most notablynatural gas. In one aspect, the invention concerns a liquified naturalgas (LNG) production system that operates with a reduced number ofprocess vessels and in a smaller space than conventional LNG productionsystems.

2. Description of the Prior Art

It is common practice to cryogenically treat natural gas to liquefy thesame for transport and storage. The primary reason for the liquefactionof natural gas is that liquefaction results in a volume reduction ofabout 1/600, thereby making it possible to store and transport theliquefied gas in containers of more economical and practical design. Forexample, when gas is transported by pipeline from a supply source to adistant market, it is desirable to operate the pipeline under asubstantially constant and high load factor. Often the deliverability orcapacity of the pipeline will exceed demand, while at other times thedemand may exceed the deliverability of the pipeline. In order to shaveoff the peaks where demand exceeds supply, it is desirable to store theexcess gas in such a manner that it can be delivered when the supplyexceeds demand, thereby enabling future peaks in demand to be met withmaterial from storage. One practical means for doing this is to convertthe gas to a liquefied state for storage and to then vaporize the liquidas demand requires.

Liquefaction of natural gas is of even greater importance in makingpossible the transport of gas from a supply source to market when thesource and market are separated by great distances and a pipeline is notavailable or is not practical. This is particularly true where transportmust be made by ocean-going vessels. Ship transportation in the gaseousstate is generally not practical because appreciable pressurization isrequired to significantly reduce the specific volume of the gas which inturn requires the use of more expensive storage containers.

In order to store and transport natural gas in the liquid state, thenatural gas is preferably cooled to −240° F. to −260° F. where itpossesses a near-atmospheric vapor pressure. Numerous systems exist inthe prior art for the liquefaction of natural gas in which the gas isliquefied by sequentially passing the gas at an elevated pressurethrough a plurality of cooling stages whereupon the gas is cooled tosuccessively lower temperatures until the liquefaction temperature isreached. Cooling is generally accomplished by heat exchange with one ormore refrigerants such as propane, propylene, ethane, ethylene, methane,or a combination of one or more of the preceding. The refrigerants aresometimes arranged in a cascaded manner. Further cooling of the liquidis possible by expanding the liquefied natural gas to atmosphericpressure in one or more expansion stages. In each stage, the liquefiedgas is flashed to a lower pressure thereby producing a two-phase,gas-liquid mixture at a significantly lower temperature. The liquid isrecovered and may again be flashed. In this manner, the liquefied gas isfurther cooled to a temperature suitable for liquefied gas storage atnear-atmospheric pressure.

As previously noted, the present invention concerns thearrangement/selection of apparatus and associated process methodologieswhereby the number of process vessels in the overall system is reduced.This reduction in the number of process vessels also reduces spacerequirements.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of this invention to reduce the number of processvessels required for liquefying natural gas.

It is another object of this invention to reduce the space requirementsof a process for liquefying natural gas.

It is still yet another object of this invention to develop a processmethodology and associated apparatus for liquefying natural gas which isless capital intensive than alternative liquefaction methodologies.

One embodiment of the present invention concerns a process forliquefying natural gas that includes the following steps: (a) cooling anatural gas stream in a first refrigeration cycle via indirect heatexchange with a first refrigerant; and (b) downstream of the firstrefrigeration cycle, cooling the natural gas stream in a secondrefrigeration cycle via indirect heat exchange with a secondrefrigerant. At least one of the first and second refrigerants is a purecomponent refrigerant, and less than about 10 percent of the natural gasmechanical cooling duty of at least one of the first and secondrefrigeration cycles is provided by core-in-kettle heat exchangers.

Another embodiment of the present invention concerns a process forliquefying natural gas that includes the following steps: (a) cooling anatural gas stream in a first refrigeration cycle employing a firstrefrigerant; (b) downstream of the first refrigeration cycle, coolingthe natural gas stream in a second refrigeration cycle employing asecond refrigerant; (c) downstream of the second refrigeration cycle,cooling the natural gas stream in a third refrigeration cycle employinga third refrigerant. The third refrigeration cycle is an openrefrigeration cycle that uses a portion of the natural gas stream as thethird refrigerant, and at least about 90 percent of the combined naturalgas mechanical cooling duty of the first, second, and thirdrefrigeration cycles is provided by plate-fin heat exchangers.

Still another embodiment of the present invention concerns a process forliquefying natural gas comprising the following steps: (a) cooling anatural gas stream in a first methane heat exchanger via indirect heatexchange with at least one predominately-methane first refrigerantstream to thereby produce a first cooled natural gas stream; (b)dividing the first cooled natural gas stream into a first refrigerantportion and a first product portion; (c) expanding the first refrigerantportion to thereby produce a first expanded refrigerant portion; and (d)using the first expanded refrigerant portion as at least a portion ofthe first refrigerant stream in the first methane heat exchanger.

Yet another embodiment of the present invention concerns a facility forproducing LNG. The facility includes the following components: (a) afirst refrigeration cycle for cooling natural gas with a firstrefrigerant; and (b) a second refrigeration cycle for cooling thenatural gas with a second refrigerant. At least one of the first andsecond refrigerants is a pure component refrigerant, and at least one ofthe first and second refrigeration cycles does not include anycore-in-kettle heat exchangers that are operable to significantly coolthe natural gas.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a simplified flow diagram of a cryogenic LNG productionprocess which illustrates one embodiment of the methodology andapparatus of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As used herein, the term “natural gas” or “natural gas stream” shalldenote any stream principally comprised of methane, which originates inmajor portion from a natural gas feed stream. A natural gas streamtypically contains at least 85 mole percent methane, with the balancebeing ethane, higher hydrocarbons, nitrogen, carbon dioxide, and minoramounts of other contaminates such as, for example, mercury, hydrogensulfide, and mercaptans. As used herein, the terms “principally,”“predominately,” “primarily,” and “in major portion,” when used todescribe the presence of a particular component of a fluid stream, shallmean that the fluid stream contains at least 50 mole percent of thestated component. For example, a “predominately” methane stream, a“primarily” methane stream, a stream “principally” comprised of methane,or a stream comprised “in major portion” of methane, each denote astream containing at least 50 mole percent methane. As used herein, theterms “upstream” and “downstream” shall be used to describe the relativepositions of various components of a natural gas liquefaction plantalong the main flow path of natural gas through the plant.

One of the most efficient and effective methodologies for natural gasliquefaction is a cascade-type operation in combination withexpansion-type cooling. Cascaded processes utilize one or morerefrigerants to transfer heat energy from the natural gas stream to therefrigerant and ultimately to the environment. In essence, therefrigeration system functions as a heat pump by removing thermal energyfrom the natural gas stream as the stream is progressively cooled tolower and lower temperatures. In so doing, the thermal energy removedfrom the natural gas stream is ultimately rejected (pumped) to theenvironment via energy exchange with one or more refrigerants.

In a preferred embodiment, the present invention employs a cascadedrefrigerant system that cools the natural gas stream at an elevatedpressure (e.g., about 650 psia), by sequentially passing the natural gasstream through an initial refrigeration cycle, an intermediaterefrigeration cycle, and a final refrigeration cycle. In a preferredembodiment of the invention, the initial and intermediate refrigerationcycles are closed refrigeration cycles, while the final refrigerationcycle is an open refrigeration cycle that utilizes a portion of the feedgas as a source of refrigerant and which includes therein a multi-stageexpansion cycle to further cool the feed gas and reduce its pressure tonear-atmospheric pressure.

The refrigerants employed in the initial, intermediate, and finalrefrigeration cycles preferably have their own distinct compositions. Inother words, it is preferred for pure component refrigerants, ratherthan mixed refrigerants, to be employed in the initial, intermediate,and final refrigeration cycles of the present invention. As used herein,the term “mixed refrigerant” denotes a refrigerant that does not containmore than 80 mole percent of any single refrigerant component. As usedherein, the term “pure component refrigerant” denotes a refrigerant thatis not a mixed refrigerant. Preferably, a pure component refrigerantcomprises at least about 80 mole percent of a single refrigerantcomponent, more preferably at least about 90 mole percent of a singlehydrocarbon refrigerant component, and most preferably at least 95 molepercent of a single hydrocarbon refrigerant component. In the system ofthe present invention, it is preferred for the refrigerant having thehighest boiling point to be utilized in the initial refrigeration cycle,followed by a refrigerant having an intermediate boiling point employedin the intermediate refrigeration cycle, and finally a refrigeranthaving the lowest boiling point is employed in the final refrigerationcycle.

In a preferred embodiment of the present invention, the initialrefrigerant employed in the initial refrigeration cycle containsprimarily propane, propylene, and/or carbon dioxide. More preferably,the initial refrigerant comprises predominately propane, mostpreferably, the initial refrigerant consists essentially of propane. Theintermediate refrigerant preferably comprises predominately ethaneand/or ethylene. More preferably, the intermediate refrigerant comprisespredominately ethylene. Most preferably, the intermediate refrigerantconsists essentially of ethylene. The final refrigerant preferablycomprises predominately methane. Most preferably, the final refrigerantconsists essentially of methane.

Preferably, each of the initial, intermediate, and final refrigerationcycles employs a plurality of distinct cooling steps carried out in oneor more heat exchangers. In a preferred embodiment of the presentinvention, less than about 10 percent of the natural gas mechanicalcooling duty of the initial, intermediate, and/or final refrigerationcycles is provided by core-in-kettle and/or spiral-wound heatexchangers, more preferably less than about 5 percent of the natural gasmechanical cooling duty of the initial, intermediate, and/or finalrefrigeration cycles is provided by core-in-kettle and/or spiral-woundheat exchangers, still more preferably less than 2 percent of thenatural gas mechanical cooling duty of the initial, intermediate, and/orfinal refrigeration cycles is provided by core-in-kettle and/orspiral-wound heat exchangers. Most preferably, none of the naturalgas-cooling heat exchangers employed in the initial, intermediate, andfinal refrigeration cycles are core-in-kettle heat exchangers and/orspiral-wound heat exchangers. Rather, it is preferred that at leastabout 90 percent of the natural gas mechanical cooling duty of theinitial, intermediate, and/or final refrigeration cycles is provided byplate-fin heat exchangers, more preferably at least about 95 percent ofthe natural gas mechanical cooling duty of the initial, intermediate,and/or final refrigeration cycles is provided by plate-fin heatexchangers, still more preferably at least 98 percent of the natural gasmechanical cooling duty of the initial, intermediate, and/or finalrefrigeration cycles is provided by plate-fin heat exchangers. Mostpreferably, all of the natural gas-cooling heat exchangers employed inthe initial, intermediate, and final refrigeration cycles are plate-finheat exchangers. It is particularly preferred for the plate-fin heatexchangers to be brazed aluminum plate-fin heat exchangers.

As used herein, the term “natural gas mechanical cooling duty” denotes aresponsibility for extracting heat from natural gas via indirect heatexchange, expressed in terms of energy per units of time (e.g., BTU/hr).As used herein, the term “core-in-kettle heat exchanger” denotes a heatexchange device comprising an outer vessel shell and an inner coredisposed in the vessel shell. A core-in-kettle heat exchangerfacilitates indirect heat transfer between a first fluid contained inthe vessel shell and a second fluid flowing through the core while thecore is at least partly submerged in the first fluid. As used here, theterm “spiral-wound heat exchanger” denotes a heat exchange devicecomprising an outer vessel shell and an inner core of wound tubesdisposed in the shell. As used herein, the term “plate-fin heatexchanger” denotes a device that defines a plurality of distinct fluidpassageways separated by plates. A plate-fin heat exchanger facilitatesindirect heat transfer between a first fluid flowing through a firstgroup of fluid passageways and a second fluid flowing through a secondgroup of fluid passageways. Heat is transferred between the first andsecond fluids via heat flux through the plates. Thus, plate-fin heatexchangers do not require the use of large containment vessels becausethe first and second fluids are contained in the fluid passagewaysduring heat transfer. As used herein, the term “brazed aluminumplate-fin heat exchanger” denotes a plate-fin heat exchanger constructedof multiple aluminum plates brazed to one another.

In a preferred embodiment of the present invention, the natural gasstream is delivered to the initial refrigeration cycle at an elevatedpressure or is compressed to an elevated pressure, that being a pressuregreater than about 500 psia, preferably about 500 to about 900 psia,still more preferably about 550 to about 675 psia, still yet morepreferably about 575 to about 650 psia, and most preferably about 600psia. The stream temperature is typically near ambient to slightly aboveambient. A representative temperature range being about 60° F. to about120° F.

Generally, the natural gas feed stream will contain such quantities ofC₂₊ components so as to result in the formation of a C₂₊ rich liquid inone or more of the cooling stages of the initial and/or intermediaterefrigeration cycles. This liquid is removed via gas/liquid separationmeans, preferably one or more conventional gas/liquid separators.Generally, the sequential cooling of the natural gas in each stage ofthe initial and/or intermediate refrigeration cycles is controlled so asto remove as much as possible of the C₂ and higher molecular weighthydrocarbons from the gas to produce a first gas stream predominating inmethane and a second liquid stream containing significant amounts ofethane and heavier components. An effective number of gas/liquidseparation means are located at strategic locations downstream of thecooling stages for the removal of liquids streams rich in C₂₊components. The exact locations and number of gas/liquid separationmeans will be dependent on a number of operating parameters, such as theC₂₊ composition of the natural gas feed stream, the desired BTU contentof the final product, the value of the C₂₊ components for otherapplications, and other factors routinely considered by those skilled inthe art of LNG plant and gas plant operation. The C₂ hydrocarbon streamor streams may be demethanized via a single stage flash or afractionation column. In the former case, the methane-rich stream can berepressurized and recycled or can be used as fuel gas. In the lattercase, the methane-rich stream can be directly returned at pressure tothe liquefaction process. The C₂₊ hydrocarbon stream or streams or thedemethanized C₂₊ hydrocarbon stream may be used as fuel or may befurther processed such as by fractionation in one or more fractionationzones to produce individual streams rich in specific chemicalconstituents (e.g., C₂, C₃, C₄ and C₅₊)

In the last cooling stage of the intermediate refrigeration cycle, theprocessed natural gas stream, which is predominantly methane (typicallygreater than 95 mole percent methane and more typically greater than 97mole percent methane), is condensed (i.e., liquefied) in major portion,preferably in its entirety. The cooled and condensed natural gas streamexiting the intermediate refrigeration cycle is then further cooled inthe final refrigeration cycle via indirect heat exchange with the finalrefrigerant. In a preferred embodiment of the present invention, thefinal refrigeration cycle is an open methane refrigeration cycleemploying a predominantly-methane refrigerant that originates from thenatural gas feed stream.

The liquefied gas entering the final refrigeration cycle preferably hasa pressure of at least about 250 psia, more preferably at least about400 psia, and most preferably in the range of from 500 to 800 psia. Itis preferred that the expansion section of the final refrigeration cycleis operable to reduce the pressure of the liquefied gas stream by atleast about 100 psi, more preferably at least about 250 psi, and mostpreferably at least 400 psi. The pressure reduction in the expansionsection of the final refrigeration cycle is preferably accomplished viaa plurality of sequential expansion steps carried out in a plurality ofexpansion devices. Each expansion device can be a Joule-Thomsonexpansion valve or a hydraulic expander. As used herein, the term“hydraulic expander” is not limited to an expander which receives andproduces liquid streams but is inclusive of expanders which receive apredominantly liquid-phase stream and produce a two-phase (gas/liquid)stream. When a hydraulic expander is employed and properly operated, thegreater efficiencies associated with the recovery of power, a greaterreduction in stream temperature, and the production of less vapor duringthe expansion step will frequently be cost-effective even in light ofincreased capital and operating costs associated with the expander. Thepressure of the liquid product entering the final refrigeration cycle ispreferably reduced to near atmospheric pressure so that the final LNGproduct has a near-atmospheric pressure and a temperature of −240° F. to−260° F.

One embodiment of the present invention provides a final refrigerationcycle having a reduced number of process vessels compare to similarrefrigeration cycles employing multi-step expansion cooling of theliquefied gas stream. In particular, in one embodiment of the presentinvention, the expansion section of the final refrigeration cycleemploys less than three vapor/liquid separation vessels (e.g., flashdrums), most preferably less than two vapor/liquid separation vessels.FIG. 1 illustrates one configuration of a final refrigeration cycle thatreduces the number of process vessels (e.g., flash drums) relative tosimilar conventional expansion-type refrigeration cycles.

The flow schematic and apparatus set forth in FIG. 1 represents apreferred embodiment of the invention employed in an open-cycle cascadedliquefaction process. Those skilled in the art will also recognized thatFIG. 1 is a schematic representation and, therefore, many items ofequipment that would be needed in a commercial plant for successfuloperation have been omitted for the sake of clarity. Such items mightinclude, for example, compressor controls, flow and level measurementsand corresponding controllers, temperature and pressure controls, pumps,motors, filters, additional heat exchangers, valves, etc. These itemswould be provided in accordance with standard engineering practice.

To facilitate an understanding of FIG. 1, items numbered 1 through 99generally correspond to process vessels and equipment directlyassociated with the liquefaction process. Items numbered 100 through 199correspond to flow lines or conduits which contain methane in majorportion. Items numbered 200 through 299 correspond to flow lines orconduits which contain the refrigerant ethylene or optionally, ethane.Items numbered 300 through 399 correspond to flow lines or conduitswhich contain the refrigerant propane.

Referring to FIG. 1, gaseous propane is compressed in multistagecompressor 18 driven by a gas turbine driver which is not illustrated.The three stages of compression preferably exist in a single unitalthough each stage of compression may be a separate unit and the unitsmechanically coupled to be driven by a single driver. Upon compression,the compressed propane is passed through conduit 300 to cooler 16 whereit is liquefied. A representative pressure and temperature of thisliquefied propane stream exiting cooler 16 is about 100° F. and about190 psia. Although not illustrated in FIG. 1, it is preferable that aseparation vessel be located downstream of cooler 16 and upstream of thehigh-stage propane brazed aluminum plate-fin heat exchanger 2, for theremoval of residual light components from the liquefied propane and toprovide surge control for the system. The refrigerant stream from thisvessel or the stream from cooler 16, as the case may be, is passedthrough conduit 302 to a high-stage propane brazed aluminum plate-finheat exchanger 2, wherein the stream flows through core passages 10 andis cooled by indirect heat exchange. The cooled or second propanerefrigerant stream is produced via conduit 303. This stream is thensplit via a splitting or separation means (illustrated but not numbered)into two portions, third and fourth propane refrigerant streams, andproduced via conduits 304 and 307. The third propane refrigerant streamflows via conduit 304 to a pressure reduction means, illustrated asexpansion valve 14, wherein the pressure of the liquefied propane isreduced, thereby evaporating or flashing a portion thereof and producinga high-stage refrigeration stream. This stream then flows throughconduit 305 and through core passages 12, wherein the stream flowscountercurrent to the stream in passage 10 and the yet to be describedstreams in passages 4, 6, and 8 and wherein indirect heat exchangeoccurs. The resulting stream, the high-stage propane recycle stream, isrouted via conduit 306 to the high-stage inlet port of propanecompressor 18. In the course of such routing, the stream will generallypass through a suction scrubber.

Also fed to plate-fin heat exchanger 2 are the natural gas stream viaconduit 100, a gaseous ethylene stream via conduit 202, and amethane-rich stream via conduit 152. These streams in flow passages 6,8, and 4 and the propane refrigerant stream in passage 10 flowcountercurrent, more preferably counterflow, to the propane stream inpassage 12. Indirect heat exchange occurs between such streams. Thestreams respectively flowing in passages 4, 6, and 8 are produced viaconduits 154, 102, and 204. The stream in conduit 204 will be referredto as a first cooled ethylene stream.

The cooled natural gas stream in conduit 102, the first cooled ethylenestream in conduit 204, and the fourth propane refrigerant stream inconduit 307 respectively flow through passages 22, 24, and 25 in brazedaluminum plate-fin heat exchanger 20 countercurrent, more preferablycounterflow, to a yet to be identified refrigeration stream therebyproducing a further cooled natural gas stream, a second cooled ethylenestream, and a fifth propane refrigerant stream which are produced viaconduits 110, 206, and 308. The fifth propane refrigerant stream is thensplit via a splitting or separation means (illustrated but not numbered)into two portions, the sixth and seventh propane refrigerant streams,and respectively produced via conduits 309 and 312. The sixth propanerefrigerant stream flows via conduit 309 to a pressure reduction means,illustrated as expansion valve 27. In expansion valve 27, the pressureof the liquefied propane is reduced, thereby evaporating or flashing aportion thereof and producing a intermediate-stage propane refrigerationstream. This stream then flows through conduit 310 and through corepassage 26 wherein said stream flows countercurrent to the streams inpassages 22, 24, and 25 and wherein indirect heat exchange occurs. Theresulting stream is produced as an intermediate-stage propane recyclestream via conduit 311. This stream is returned to theintermediate-stage inlet port of propane compressor 18, again preferablyafter passing through a suction scrubber.

The further cooled natural gas stream and the second cooled ethylenestream are respectively routed via conduits 110 and 206 to respectivepassages 36 and 38 in brazed aluminum plate-fin heat exchanger 34wherein the natural gas stream is yet further cooled. The natural gasand ethylene streams are produced from plate-fin heat exchanger 34 viaconduits 112 and 208, respectively.

The seventh propane refrigerant stream in conduit 312 is connected tobrazed aluminum plate-fin heat exchanger 28 wherein the stream flows viapassage 29 countercurrent, more preferably counterflow, to and inindirect heat exchange with a low-stage propane refrigerant flowing viapassage 30 thereby producing an eighth propane refrigerant stream viaconduit 314. The eighth propane refrigerant flows via conduit 314 to apressure reduction means, illustrated as expansion valve 32, wherein thepressure of the liquefied propane is reduced thereby evaporating orflashing a portion thereof and producing a two-phase refrigerant stream.The expanded refrigerant stream is carried to brazed aluminum plate-finheat exchanger 34 where it is employed as a cooling agent in passage 37.A low-stage propane refrigeration stream is removed from heat exchanger34 via conduit 318. This conduit is connected to passage 30 in heatexchanger 28 wherein said stream flows countercurrent and is in indirectheat exchange with the seventh propane refrigerant stream in passage 29thereby producing a low-stage propane recycle stream. The low-stagepropane recycle stream is then returned to the low-stage inlet port ofcompressor 18, preferably after flow through a suction scrubber, viaconduit 320. In compressor 18, the low-stage propane recycle stream iscompressed, combined with the intermediate-stage propane recycle stream,and compressed to form a compressed intermediate-stage recycle stream.This stream is then combined with the high-stage propane recycle streamto form a combined high-stage propane recycle stream which is compressedto form the compressed propane refrigerant stream produced via conduit300.

In one embodiment of the invention, the brazed aluminum plate-fin heatexchangers 2, 20, 28, and 34 of the initial (propane) refrigerationcycle are separate heat exchangers. In another embodiment, the heatexchangers are combined into one or more exchangers. Although resultingin a more complex heat exchanger which possesses intermediate headers,combined approach can offer advantages from a lay-out and costperspective.

In the intermediate refrigeration cycle depicted in FIG. 1, the naturalgas stream is cooled via indirect heat exchange with an ethylenerefrigeration stream until it is substantially condensed. As illustratedin FIG. 1, a low-stage ethylene recycle stream delivered via conduit 232is compressed in compressor 40 and the resulting compressed low-stageethylene recycle stream is preferably removed from compressor 40 viaconduit 234, cooled via inter-stage cooler 71, returned to thecompressor via conduit 236 and combined with a high-stage ethylenerecycle stream delivered via conduit 216 whereupon the combined streamis compressed to thereby producing a compressed ethylene refrigerantstream via conduit 200. A preferred pressure for this compressedethylene refrigerant stream is approximately 300 psia. Preferably, thetwo compressor stages of compressor 40 are a single module although theymay each be a separate module and the modules mechanically coupled to acommon driver. The compressed ethylene refrigerant stream is routed fromthe compressor 40 to the downstream cooler 72 via conduit 200. Theproduct from cooler 72 flows via conduit 202 and is introduced, aspreviously discussed, to the initial refrigeration cycle wherein thestream is further cooled and liquefied via heat exchange passages 8, 24,and 38 and then returned to the intermediate refrigeration cycle viaconduit 208. This stream in conduit 208 preferably flows to a separationvessel 41 which provides for the removal of residual light componentsfrom the liquefied stream and which also provides surge volume for therefrigeration system. A refrigerant stream, referred to herein withregard to the intermediate refrigeration cycle as a first ethylenerefrigerant stream, is produced from vessel 41 via conduit 209.

The cooled natural gas stream produced from the initial refrigerationcycle via conduit 112 is combined with a yet to be describedmethane-rich stream provided via conduit 156. This combined stream inconduit 114 and the first refrigerant ethylene stream in conduit 209 arerouted to a brazed aluminum plate fin-heat exchanger 42 wherein thesestreams flow through core passages 44 and 46 countercurrent, morepreferably counterflow, to and in indirect heat exchange with a yet tobe described high-stage ethylene refrigerant stream and optionally, alow-stage ethylene refrigerant stream respectively flowing in passages48 and 50. A cooled stream referred to herein as a second ethylenerefrigerant stream is produced from passage 46 via conduit 210. Thisstream is then split via a splitting or separation means (illustratedbut not numbered) into two portions, third and fourth ethylenerefrigerant streams, and produced via conduits 212 and 218. The thirdethylene refrigerant stream flows via conduit 212 to a pressurereduction means, illustrated as expansion valve 52, wherein the pressureof the liquefied ethylene is reduced thereby evaporating or flashing aportion thereof and producing a high-stage ethylene refrigerationstream. This stream then flows through conduit 214 and through corepassage 48 thereby producing a high-stage ethylene recycle stream whichis transported via conduit 216 to the high-stage inlet port ofcompressor 40.

A further cooled natural gas stream is produced from passage 44 viaconduit 116 and is optionally combined with a methane-rich recyclestream delivered via conduit 158. The resulting stream is routed viaconduit 120 to a passage 59 of a brazed aluminum plate-fin heatexchanger 58 wherein the stream is cooled and liquefied in major portionand the resulting stream is produced via conduit 122.

The fourth ethylene refrigerant stream is transported via conduit 218 toa passage 54 in a brazed aluminum plate-fin heat exchanger 53. Thefourth ethylene refrigerant stream flows countercurrent, more preferablycounterflow, to and is in indirect heat exchange with a low-stageethylene refrigerant flowing via passage 55 in heat exchanger 53,thereby producing a fifth ethylene refrigerant stream via conduit 220.The fifth ethylene refrigerant stream flows via conduit 220 to apressure reduction means, illustrated as expansion valve 56, wherein thepressure of the liquefied ethylene is reduced, thereby evaporating orflashing a portion thereof and producing a two-phase ethylenerefrigerant stream. The resulting two-phase ethylene refrigerant streamis carried via conduit 226 to heat exchanger 58 wherein the stream isemployed as a cooling agent in passage 57. A low-stage ethylenerefrigeration stream is removed from heat exchanger 58 via conduit 228.Conduit 228 is connected to passage 55 in heat exchanger 53 wherein saidstream flows countercurrent and is in indirect heat exchange with thefluid in passage 54 thereby producing a low-stage ethylene recyclestream. This stream is returned to the low-stage inlet port ofcompressor 40 via conduit 232. Optionally, and as depicted in FIG. 1,this stream may also flow to plate-fin heat exchanger 42 via conduit 230and through passage 50 wherein said stream flows countercurrent, morepreferably counterflow, to the fluids in passages 44 and 46 and isfurther warmed prior to flow to the compressor 40 via conduit 232.

In one embodiment of the invention, brazed aluminum plate-fin heatexchangers 42, 53, and 58 of the intermediate refrigeration cycle areseparate heat exchangers. In another embodiment, the heat exchangers arecombined into a single exchanger.

The liquefied natural gas stream produced from plate-fin heat exchanger58 via conduit 122 is generally at a temperature of about −125° F. and apressure of about 600 psi. The liquefied stream in conduit 122 isintroduced into the final refrigeration cycle where it undergoes coolingby indirect heat exchange with a methane refrigerant and by expansion.The stream in conduit 122 is initially cooled in a main methaneeconomizer 74 via indirect heat exchange with methane refrigerantstreams in passages 82, 95, and 96. In a preferred embodiment of thepresent invention, the methane refrigerant employed in the finalrefrigeration cycle is derived from the processed natural gas stream,thereby making the final refrigeration cycle an open methanerefrigeration cycle. Main methane economizer 74 is preferably aplate-fin heat exchanger, most preferably a brazed aluminum plate-finheat exchanger. The liquefied natural gas stream introduced into mainmethane economizer 74 via conduit 122 is cooled in passage 76 and thenexits main methane economizer 74 via conduit 124. The cooled stream inconduit 124 is subsequently divided into a first refrigerant portioncarried in conduit 125 and a first product portion carried in conduit126. The first refrigerant portion in conduit 125 is transported to anexpansion means (illustrated as expansion valve 78), wherein the streamis reduced in pressure to thereby produce a first expanded refrigerantportion in conduit 127. The first expanded refrigerant portion inconduit 127 is then introduced into passage 82 of main methaneeconomizer 74 wherein it is employed as a refrigerant to cool thenatural gas stream in passage 76. The warmed first refrigerant streamexits passage 82 and methane economizer 74 via conduit 128, and isintroduced into the high-stage inlet port of methane compressor 83.

The first product portion in conduit 126 is carried to a second methaneeconomizer 87 for further cooling. The second methane economizer 87 ispreferably a plate-fin heat exchanger, most preferably a brazed aluminumplate-fin heat exchanger. In second methane economizer 87, the firstproduct portion is cooled as it passes through passage 88 and indirectlyexchanges heat with the refrigerant streams passing through passages 89and 90, described in more detail below. A second cooled natural gasstream is produced from second methane economizer 87 via conduit 129.The second cooled natural gas stream in conduit 129 is subsequentlydivided into a second refrigerant portion carried in conduit 130 and asecond product portion carried in conduit 131. The second refrigerantportion carried in conduit 130 is subsequently expanded in expansionvalve 91 to thereby produce a second expanded refrigerant portion. Thesecond product portion in conduit 131 is subsequently expanded inexpansion valve 92 to thereby produce a two-phase second stream that issubsequently carried to a phase separator 93 via conduit 132. The secondexpanded refrigerant portion expander 91 is transported via conduit 133to second methane economizer 87 wherein the second expandedrefrigeration portion is employed as a refrigerant in passage 89 to coolthe stream flowing in passage 88. After being employed as a coolingagent in passage 89, the warmed second refrigerant portion is removedfrom second methane economizer 87 via conduit 134 and subsequentlyintroduced into a passage 95 of main methane economizer 74 wherein thewarmed second refrigerant portion is used to cool the stream in passage76. The further warmed second refrigerant portion exits methaneeconomizer 74 via conduit 135 and is subsequently introduced into theintermediate-stage inlet port of methane compressor 83.

The two-phase in conduit 132 is separated in vapor/liquid separator 93to thereby produce a gaseous third refrigerant portion via conduit 136and a liquid third product portion via conduit 142. The gaseous thirdrefrigerant stream in conduit 136 is combined with the compressed streamin conduit 138, described in further detail below. The resultingcombined stream flows via conduit 139 to second methane economizer 87wherein the combined stream is employed as a refrigerant in passage 90to cool the stream in passage 88. The warmed third portion exits passage90 of second methane economizer 87 via conduit 140 and is carried topassage 96 of main methane economizer 74 wherein the refrigerant streamis used to cool the stream in passage 76. The further warmed thirdrefrigerant portion exits passage 96 and main methane economizer 74 viaconduit 141 and is passed to the low-stage inlet port of methanecompressor 83.

The liquid third product portion that exits separator 93 via conduit 142is expanded in expansion valve 94 to thereby produce a two-phaseexpanded third product stream which is carried to LNG storage tank 99via conduit 143. The vapor portion of the stream introduced in to LNGstorage tank 99 and any boil-off vapors generated in tank 99 are removedfrom tank 99 via conduit 144. This vapor stream in conduit 144 iscompressed in compressor 96 to produce the compressed gas stream inconduit 138 that is subsequently combined with the separated vaporstream in conduit 136 before being employed as a refrigerant in thesecond methane economizer 87 and the main methane economizer 74. The LNGin tank 99 can be stored and subsequently transported to a distantmarket where it is gasified for use as an energy source.

As shown in FIG. 1, the three stages of compression provided by methanecompressor 83 are preferably contained in a single unit. However, eachcompression stage may exist as a separate unit where the units aremechanically coupled together to be driven by a single driver. Thecompressed gas from the low-stage section of compressor 83 preferablypasses through an inter-stage cooler 85 and is combined with theintermediate pressure gas in conduit 140 prior to the second-stage ofcompression. The compressed gas from the intermediate stage ofcompressor 83 is preferably passed through an inter-stage cooler 84 andis combined with the high pressure gas in conduit 140 prior to the thirdstage of compression. The compressed gas is discharged from thehigh-stage methane compressor through conduit 150, is cooled in cooler86 and is routed to the high-stage propane chiller 2 via conduit 152.The methane-rich stream exiting chiller 2 via conduit 154 is fed to mainmethane economizer 74 wherein the stream is cooled via indirect heatexchange with one or more of the streams in passages 82, 94, and/or 96.In one embodiment and as illustrated in FIG. 1, the stream delivered viaconduit 154 is cooled in the main methane economizer 74 via indirectheat exchange means 97, a portion removed via conduit 156 and theremaining stream further cooled via indirect heat exchange means 98 andproduced via conduit 158. This is a preferred embodiment. In this splitstream embodiment, a portion of the compressed methane recycle streamdelivered via conduit 156 is combined with the natural gas stream viaconduit 112 immediately upstream of the intermediate refrigeration cycleand the remaining portion delivered via conduit 158 combined with thestream in conduit 116 immediately upstream of brazed aluminum plate-finheat exchanger 58 wherein the majority of liquefaction of the naturalgas stream occurs. In a simpler embodiment (i.e., less preferred from aprocess efficiency perspective), the methane recycle stream is cooled inits entirety in the main methane economizer 74 and combined via conduit158 with the natural gas stream in conduit 112 immediately upstream ofthe second cycle.

With regard to the compressor/driver units employed in the process, FIG.1 depicts individual compressor/driver units (i.e., a single compressiontrain) for the propane, ethylene and open-cycle methane compressionstages. However in a preferred embodiment for any cascaded process,process reliability can be improved significantly by employing amultiple compression train comprising two or more compressor/drivercombinations in parallel in lieu of the depicted singlecompressor/driver units. In the event that a compressor/driver unitbecomes unavailable, the process can still be operated at a reducedcapacity.

In one embodiment of the present invention, the LNG production system ofFIG. 1 is simulated on a computer using conventional process simulationsoftware. Examples of suitable simulation software include HYSYS™ fromHyprotech, Aspen Plus® from Aspen Technology Inc., and PRO\II® fromSimulation Sciences Inc.

While specific cryogenic methods, materials, items of equipment andcontrol instruments are referred to herein, it is to be understood thatsuch specific recitals are not to be considered limiting but areincluded by way of illustration and to set forth the best mode inaccordance with the present invention.

1. A process for liquefying natural gas, said process comprising: (a)cooling a natural gas stream in a first refrigeration cycle via indirectheat exchange with a first refrigerant; and (b) downstream of step (a),cooling said natural gas stream in a second refrigeration cycle viaindirect heat exchange with a second refrigerant, wherein at least oneof said first and second refrigerants is a pure component refrigerant,wherein less than about 10 percent of the natural gas mechanical coolingduty of at least one of said first and second refrigeration cycles isprovided by core-in-kettle heat exchangers.
 2. The process of claim 1wherein less than about 5 percent of the natural gas mechanical coolingduty of said first and second refrigeration cycles is provided bycore-in-kettle heat exchangers.
 3. The process of claim 1 wherein atleast about 90 percent of the natural gas mechanical cooling duty ofsaid first and second refrigeration cycles is provided by one or moreplate-fin heat exchangers.
 4. The process of claim 3 wherein saidplate-fin heat exchangers are brazed aluminum plate-fin heat exchangers.5. The process of claim 1 wherein said second refrigerant is a purecomponent refrigerant.
 6. The process of claim 1 wherein said first andsecond refrigerants are both pure component refrigerants.
 7. The processof claim 1 wherein said first refrigerant comprises predominatelypropane and said second refrigerant comprises predominately ethylene. 8.The process of claim 1 further comprising, downstream of step (b),cooling said natural gas stream in a third refrigeration cycle viaindirect heat exchange with a third refrigerant.
 9. The process of claim8 wherein said third refrigerant is a pure component refrigerant. 10.The process of claim 8 wherein said third refrigeration cycle is an openrefrigeration cycle employing a portion of said natural gas stream assaid third refrigerant.
 11. The process of claim 8 wherein said first,second, and third refrigerants are all pure component refrigerants. 12.A process for liquefying natural gas comprising: (a) cooling a naturalgas stream in a first refrigeration cycle employing a first refrigerant;(b) downstream of step (a), cooling the natural gas stream in a secondrefrigeration cycle employing a second refrigerant; (c) downstream ofstep (b), cooling the natural gas stream in a third refrigeration cycleemploying a third refrigerant; wherein said third refrigeration cycle isan open refrigeration cycle that uses a portion of said natural gasstream as said third refrigerant, wherein at least about 90 percent ofthe combined natural gas mechanical cooling duty of said first, second,and third refrigeration cycles is provided by plate-fin heat exchangers.13. The process of claim 12 wherein said cooling of steps (a), (b), and(c) is carried out without the use of any core-in-kettle heat or spiralwound exchangers.
 14. The process of claim 12 wherein all of saidcooling of steps (a), (b), and (c) is carried in plate-fin heatexchangers.
 15. The process of claim 12 further comprising, cooling saidnatural gas stream via expansion in said third refrigeration cycle. 16.The process of claim 15 wherein the pressure of said natural gas streamis reduced by at least about 100 psi in said third refrigeration cycle.17. The process of claim 15 wherein said natural gas stream enters saidthird refrigeration cycle at a pressure of at least about 400 psia,wherein the pressure of said natural gas stream is reduced by at least250 psi in said third refrigeration cycle.
 18. The process of claim 12wherein said first refrigerant comprises predominately propane and/orpropylene, said second refrigerant comprises predominately ethane and/orethylene, and said third refrigerant comprises predominately methane.19. A process for liquefying natural gas comprising: (a) cooling anatural gas stream in a first methane heat exchanger via indirect heatexchange with at least one predominately-methane first refrigerantstream to thereby produce a first cooled natural gas stream; (b)dividing said first cooled natural gas stream into a first refrigerantportion and a first product portion; (c) expanding said firstrefrigerant portion to thereby produce a first expanded refrigerantportion; and (d) using said first expanded refrigerant portion as atleast a portion of said first refrigerant stream in said first methaneheat exchanger.
 20. The process of claim 19 wherein said first expandedrefrigerant portion is not subjected to phase separation prior to beingused as said first refrigerant stream in said first methane heatexchanger.
 21. The process of claim 19 further comprising, cooling saidfirst product portion in a second methane heat exchanger via indirectheat exchange with at least one predominately-methane second refrigerantstream to thereby produce a second cooled natural gas stream.
 22. Theprocess of claim 21 further comprising, dividing said second coolednatural gas stream into a second refrigerant portion and a secondproduct portion.
 23. The process of claim 22 further comprising,expanding said second refrigerant portion to thereby produce a secondexpanded refrigerant portion and using said second expanded refrigerantportion as at least a portion of said second refrigerant stream in saidsecond methane heat exchanger.
 24. The process of claim 23 wherein saidsecond expanded refrigerant portion is not subjected to phase separationprior to being used as said second refrigerant stream in said secondmethane heat exchanger.
 25. The process of claim 23 wherein said secondexpanded refrigerant portion is heated in said second methane heatexchanger to thereby produce a warmed second expanded refrigerantportion, wherein said process further comprises employing said warmedsecond expanded refrigerant portion as at least a portion of said firstrefrigerant stream in said first methane heat exchanger.
 26. The processof claim 22 further comprising, expanding said second product portion tothereby produce a two-phase stream comprising a vapor third refrigerantportion and a liquid third product portion and using said thirdrefrigerant portion as at least a portion of said second refrigerantstream in said second methane heat exchanger.
 27. The process of claim26 wherein said third refrigerant portion is heated in said secondmethane heat exchanger to thereby produce a warmed third refrigerantportion, wherein said process further comprises using said warmed thirdrefrigerant portion as at least a portion of said first refrigerantstream in said first methane heat exchanger.
 28. The process of claim 26further comprising, expanding said third product portion to therebyproduce an expanded final product stream comprising liquified naturalgas and introducing at least a portion of said expanded final productstream into an LNG storage tank.
 29. The process of claim 28 furthercomprising, removing vapors from said LNG storage tank and employingsaid vapors as at least a portion of said first and/or secondrefrigerant streams.
 30. The process of claim 19 further comprising,prior to step (a) cooling said natural gas stream via indirect heatexchange with an initial refrigerant stream, wherein said initialrefrigerant stream comprises predominately propane, propylene, ethane,and/or ethylene.
 31. The process of claim 30 wherein said initialrefrigerant stream comprises predominately propane.
 32. The process ofclaim 31 further comprising, prior to step (a) cooling said natural gasstream via indirect heat exchange with intermediate refrigerant stream,wherein said intermediate refrigerant stream comprises predominatelyethane and/or ethylene.
 33. A process comprising: gasifying LNG producedby the process of claim
 1. 34. A process comprising: gasifying LNGproduced by the process of claim
 12. 35. A process comprising: gasifyingLNG produced by the process of claim
 19. 36. A computer simulationprocess comprising: using a computer to simulate the process of claim 1.37. A computer simulation process comprising: using a computer tosimulate the process of claim
 12. 38. A computer simulation processcomprising: using a computer to simulate the process of claim
 19. 39. Afacility for processing natural gas into producing LNG, said facilitycomprising: a first refrigeration cycle for cooling natural gas with afirst refrigerant; and a second refrigeration cycle for cooling saidnatural gas with a second refrigerant, wherein at least one of saidfirst and second refrigerants is a pure component refrigerant, whereinat least one of said first and second refrigeration cycles does notcomprise any core-in-kettle heat exchangers that are operable tosignificantly cool said natural gas.
 40. The facility of claim 39wherein said first and second refrigeration cycles do not comprise anycore-in-kettle or spiral-wound heat exchangers that are operable to coolsaid natural gas.
 41. The facility of claim 39 wherein said first andsecond refrigeration cycles include a plurality of plate-fin heatexchangers that are operable to cool said natural gas.
 42. The facilityof claim 39 wherein said first and second refrigerants are both purecomponent refrigerants.
 43. The facility of claim 39 wherein said firstrefrigerant comprises predominately propane and/or propylene and saidsecond refrigerant comprises predominately ethane and/or ethylene. 44.The facility of claim 39 further comprising, a third refrigeration cyclefor cooling said natural gas with a third refrigerant.
 45. The facilityof claim 44 wherein said third refrigerant comprises predominatelymethane.
 46. The facility of claim 44 wherein said third refrigerationcycle is an open refrigeration cycle.
 47. The facility of claim 39wherein said third refrigeration cycle includes a plurality of expansiondevices for sequentially lowering the pressure of said natural gas,wherein said third refrigeration cycle employs less than threevapor/liquid separation vessels.
 48. The facility of claim 47 whereinsaid third refrigeration cycle employs less than two vapor/liquidseparation vessels.